1. Field of the Invention
The present invention relates, generally, to the field of wireless communications, and, more specifically, to low power local area wireless communications such as point to point data communications, automatic identification, remote control, and status reporting.
2. Description of the Related Art
The radio link performance for equipment like computer to computer data links, garage door openers, vehicle toll tags, and remote keyless entry systems is often limited by the high emphasis placed upon small size and low cost. Progress in equipment design in this field requires advances in performance where the necessary low cost and size are still maintained.
The earliest equipment of this type involved amplitude modulated transmitters based on free running oscillators, usually inductor/capacitor (LC) resonator based. Receivers were often of the low part count regenerative type, also using free running LC oscillators. These LC systems were of very low performance, and the desire to improve system operation led to general industry conversion to surface acoustic wave (SAW) based technology, beginning in the early 1980's. SAW's are a form of crystal resonator where the resonant acoustic vibration that defines crystal operating frequency is constrained to the surface region of the device material, as opposed to classic bulk mode crystals, where the acoustic vibration occurs throughout the body of the device. The surface mode of operation allows operation to much higher frequencies than bulk mode, extending currently to several GHz, but at the cost of reduced frequency accuracy as compared to bulk mode crystals. Though much more frequency accurate than LC technology, SAW's exhibit approximately one to two orders of magnitude more frequency error than bulk mode crystals. This frequency error leads SAW based receivers and transmitters to typically employ a wide band channel bandwidth that is much wider than the modulation bandwidth of the transmitted signal, which in turn worsens sensitivity, range, and immunity to interference as opposed to a narrower bandwidth that did not allow excessive noise contamination of the signal.
For example, in the 902 to 928 MHz Industrial, Scientific, and Medical (ISM) band, the frequency accuracy of a SAW based transmitter is typically+/-100 KHz with a typical data rate of 5000 bits/second and a transmitted bandwidth of approximately 10 KHz. The 200 KHz minimum receiver bandwidth to have this transmitter come up in channel is 20 times greater than the bandwidth required by the data rate. This leads to reduction in sensitivity of approximately 13 dB and a twenty times increase in vulnerability to interference compared to a minimum bandwidth receiver design. However, an advantage of having a receive channel bandwidth wide enough to cover any possible frequency error in the transmitter is that the receiver can very quickly detect the incoming signal and prepare itself for actual data reception.
At the same time that SAW's have come to dominate low power wireless, the cellular telephone and higher level wireless industry have achieved great market success, which has in turn led to a new generation of high performance yet compact radio technology. The frequency sources for this class of equipment are generally processor controlled frequency synthesizers, which have been steadily increasing in integration level, to the point where a synthesized signal source can be competitive in size to a SAW based signal source. These compact synthesizers allow transmitters and channel defining local oscillators in receivers to be set via software control to any of a large number of frequencies uniformly distributed across the band of interest. The frequency accuracy is determined by precision bulk mode crystal oscillator references, which allows channel placement error to be constrained to a small fraction of the bandwidth of typical data rates, thus eliminating the need for wider receiver bandwidth to allow for frequency error. Since electronic noise that limits receiver sensitivity is directly proportional to bandwidth, using the narrowest possible bandwidth leads to maximum sensitivity. This more advanced technology can be applied to some segments of the low power wireless market directly, but in other segments the demands for the lowest possible power, size, and expense cannot easily be met by these more sophisticated methods. A highly accurate crystal based reference can be more expensive than an entire transmitter in a low power wireless application. Therefore a synthesized transmitter in such an application would typically use a lower accuracy crystal reference that, while superior in accuracy to a SAW device, still causes frequency error that is a large fraction of the desired channel bandwidth. Maintaining optimum system performance with the reduced transmit frequency accuracy of either SAW or inexpensive crystals requires specialized receiver techniques aimed at maintaining minimum receiver bandwidth in the presence of the frequency error, and minimum time latency in dealing with such frequency error. However, specialized receiver designs optimized for this type of operation are not commercially available.
Therefore, there exists a need for an apparatus and a method which provides the higher performance available from optimum narrow band systems utilizing high accuracy crystal reference based frequency stabilization while maintaining, to the greatest extent possible, the low cost and low size advantages of the SAW based systems.